Caption: Ultrasound scan of leg. Patient undergoing a Doppler ultrasound (angiodynography) scan to study blood flow and explore potential deep vein thrombosis. The display (upper right) shows the area being studied. The transducer being held against the leg sends high frequency sound waves and picks up their echoes. The Doppler effect is used to distinguish between the flow of venous and arterial blood. A thrombus (clot) can form in the deep veins of the leg, which can restrict blood flow. This can cause pain and swelling. This may be due to age or a lack of mobility for long periods. Deep vein thrombosis has been linked to long haul air travel.